Field of the Invention
Exemplary embodiments of the present invention relate to a brushless motor, and more particularly, to a brushless motor in which a rotor is rotated around a stator and a permanent magnet is provided on the rotor.
Background of the Invention
Generally, a motor is a device which converts electric energy into mechanical energy in a magnetic field in which electric current flows. Motors can be classified into various types depending on a variety of criteria such as the kind of power, the positions of a rotor and a stator, whether a permanent magnet is present or not, etc.
For example, motors may be classified into a direct current (DC) motor and an alternating current (AC) motor according to the kind of power. DC motors are also classified into a brushed motor and a brushless motor.
A brushed motor of DC motors allows current to flow through a coil by means of contact between a commutator and a brush and has a function of commutating the current, but it is disadvantageous in that mechanical or electrical noise is generated and the brush is worn. In an effort to overcome the above-mentioned disadvantages, BLDC (brushless DC) motors having no brush have been widely used. Such a BLDC motor is a DC motor that has neither brush nor commutator but includes an electronic commutation unit, and is also called a commutatorless motor.
Furthermore, motors may be classified into an inner-rotor motor and a brushless motor according to a relative position of a rotor and a stator. FIGS. 1 and 2 illustrate an example of a BLDC motor.
A BLDC motor illustrated in FIGS. 1 and 2 includes a rotor 110 in which permanent magnets 112 are provided on the inner circumferential surface of a rotor core 111, and a stator 120 in which teeth 122 are provided on outer ends of respective stator cores 121 around each of which a coil 125 is wound.
Meanwhile, when the motor rotates, the magnitude of magnetic resistance, which impedes the flow of magnetic flux, varies depending on the position of the rotor 110. Due to such variation in magnetic resistance, torque ripple is caused. As such, torque generated when the rotor rotates before power is applied to the coil refers to cogging torque. Such cogging torque results in vibration and noise.
The magnitude of cogging torque is known as being proportional to a rate of variation in magnetic resistance in response to variation in position of the rotor. To reduce such cogging torque, in each of the teeth 122 according to the conventional technique shown in FIGS. 1 and 2, a notch 124 is formed in a facing surface 123 that faces the permanent magnet 112.
Each of the teeth 122 is an element which extends in the circumferential direction of the rotor core 111 such that magnetic flux of the stator 120 is transferred to the rotor 110. A plurality of notches 124 are formed in the facing surface 123 of each tooth 122 and arranged in the circumferential direction of the rotor core 111.
However, even through the notches 124 are formed in each tooth 122, as shown in FIG. 5, the variation rate of the magnetic resistance in response to the rotation angle of the rotor is still comparatively large. Thus, there is a problem in that noise and vibration cannot be satisfactorily reduced.